The present invention relates to an acoustic image localization apparatus of an electronic musical instrument. The acoustic image localization apparatus is particularly applicable to the electronic musical instrument having a multiple of sound sources.
An audio system conventionally adopts an acoustic image localization called "binaural method". An audio signal recorded by the binaural method is reproduced through a headphone to thereby realize presence of a concert hall and to thereby form a sophisticated sound field. Recently, an electronic musical instrument also adopts localization of a performance sound to achieve a more realistic presence.
An acoustic image localization apparatus of the conventional electronic musical instrument will be described briefly in conjunction with FIGS. 10 and 11. FIG. 10 schematically illustrates a measurement of a transfer function at a pair of ears formed on a dummy head 101 within a room 104. In this example, based on the symmetry of right and left ears with respect to a center of the head, a microphone 102 is set alone into the right ear to measure a transfer function with respect to not only the right ear but also the left ear. As well known, the transfer function is given as a Laplace transform of an impulse response. Therefore, a sound source is utilized to emit an impulse toward the ear so as to readily measure the transfer function. In detail, the sound source 103 is positioned to a selected one of predetermined points as indicated by circles to emit an impulse of sound, and the microphone 102 attached to the right ear of the dummy head 101 receives the emitted impulse, which is analyzed to determine the transfer function associated to the selected point. The impulse sound source 103 is successively displaced to other points, thereby determining transfer functions of the right ear with respect to all the points which are provisionally determined throughout the listening room 104. With regard to the left ear, in view of the symmetry of the right and left ears with respect to the center line of the room 104, a transfer function of the left ear at one point is given as a measured transfer function of the right ear at another point which is symmetric to said one point with respect to the center line.
Referring to FIG. 11, the disclosed apparatus operates to localize an acoustic image of musical sounds with using the thus obtained transfer functions. The apparatus is provided with a tone generator 121 which generates a monaural signal in response to events played on a keyboard 120. A table memory 124 is provided for storing a data of the provisionally measured transfer functions in the form of finite impulse response (FIR) parameters. A pair of right and left FIR filters 122, 123 receive the monaural signal fed from the tone generator 121. Each of the FIR filters 122, 123 is set with a particular FIR parameter which is specified according to acoustic image localization coordinates X, Y. The FIR filters 122, 123 function as an electric simulator of the transfer function so as to filter the monaural signal according to the set FIR parameter. Additionally, an interpolator 125 is disposed between the table memory 124 and each of the FIR filters 122, 123 to interpolatively calculate the FIR parameter in case that the acoustic image localization coordinates X, Y specify an intermediate point which falls between the fixed points marked by the circles in FIG. 10. Namely, the interpolator 125 calculates the FIR parameter by interpolation of the stored FIR, parameters corresponding to the fixed points in the vicinity of the intermediate point. A left amplifier 126 amplifies an output signal subjected to the acoustic image localization by the FIR filter 122 to produce a left channel signal. A right amplifier 127 amplifies another output signal subjected to the acoustic image localization by the FIR filter 123 to produce a right channel signal. The pair of the left and right channel signals are inputted into a headphone (not shown) to reproduce a stereo sound which has an acoustic image localized to the specified point.
However, the conventional acoustic image localization apparatus can localize only one acoustic image. Therefore, the conventional apparatus hardly realize a presence as if a multiple of sound sources are widely distributed throughout a sound field likewise an orchestra. Further, the conventional apparatus localizes an acoustic image with respect to a sole listening point, thereby restricting optimum positioning of a listener. Additionally in the measurement system of the transfer function shown in FIG. 10, a positional error may be produced between left hand points and righthand points in practical reasons. Due to this positional error, the microphone attached to the dummy head may receive a somewhat shifted impulse response. Consequently, the measured transfer function is also irregularly shifted. Therefore, the conventional apparatus utilizing the shifted transfer function may localize the acoustic image in a rather vague form. Even worse, a stereo sound is not obtained but a monaural sound emerges when the acoustic image is localized just in front or back of the dummy head, because the transfer function measured with respect to the right ear is diverted to a transfer function of the left ear without actual measurement.